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Chemical Process Technology

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Sunday, March 1, 2009

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Recently there have been many discussion and argument among engineers from operating company, design and engineering, manufacturer and consulting on one simple but important term relates to Pressure Relief Device (PRD), "backpressure". A lot of terms have been used and mixed and lead to confusion. Beside, there are also some new terms created unintentionally which further confuse the discussion. This post is intended to clarify common definition used in Chemical Process Technology

Several definition of "backpressure" have been pointed out in the discussion :

i) Backpressure
ii) Superimposed backpressure
iii) Built-up backpressure
iv) Total backpressure
v) Constant backpressure
vi) Variable backpressure
vii) Accumulated backpressure
viii) Total accumulated backpressure
All these definition have been used in the discussion and lead to confusion and conflict.

Correct Definition and Combination
Every companies and engineers may have different definition and understanding about "backpressure". The combination of terms used within a code or company may be correct. However, one specific term used in one code or company shall not mix with another term used in another code or company.

For example, instrument datasheet is used to transfer and communicate information between an instrument engineer in design company and engineer in PRD manufacturer. The terms used possibly are Constant backpressure, Variable backpressure and Total backpressure. However, a process engineer in design company may use Superimposed backpressure, Built-up backpressure and Backpressure to transfer and communicate similar informations to instrument engineer in design company. Whenever the discussion among three parties (process engineer, instrument engineer & manufacturer engineer), correct combination of terms shall be clarified and used. Confusion will occur when different combination are used e.g. built-up backpressure mix with constant and variable backpressure.

Common Definition of "Backpressure" in Chemical Process Technology
A pressure relief valve (PRV) in "Ready-to-operate" mode and "relieving" mode during plant operation will expose to different type of pressure. "Ready-to-operate" mode is the PRV's disc keeping PRV in closed position when inlet pressure (Pi) is lower than or equal to PRV set pressure (Ps). "Relieving" mode is PRV disc away from seat allowing fluid passing the PRV valve nozzle when the inlet pressure (Pi) is higher than PRV set pressure (Ps).

As PRV may be in Ready-to-operate and Relieving mode, the "backpressure" exist at different modes will vary. In many event, understanding of this "backpressure" creates a lot of confusion among engineers in operation, design, vendor, manufacturer, etc. It is important to make the defintion clear prior to any discussion.

Superimposed backpressure (Pbs) is the static pressure that exists at the outlet of a pressure relief device (PRD) at the time the device is required to operate ("Ready-to-operate" mode). Superimposed backpressure is the result of pressure in the discharge system coming from other sources i.e. Pressure control valve (PCV), pressure relief valve (PRV), etc. Superimposed backpressure may be constant or variable.

Built-up backpressure (Pbb) is the increase in pressure at the outlet of a pressure relief device that develops as a result of flow after the pressure relief device (PRD) opens ("relieving" mode).

Backpressure (Pb) is the pressure that exists at the outlet of a pressure relief device (PRD) as a result of the pressure in the PRD discharge system. Backpressure is pressure result of both superimposed backpressure caused by other sources and built-up backpressure due to the relief flow during "relieving" mode. Backpressure is the sum of the superimposed and built-up backpressures.

Pb = Pbs + Pbb

Above defintion are inline with API STD 520 Part 1, Dec 2008 - Sizing, Selection & Installation of Pressure-Relieving Devices in Refinery.

"Backpressure" will affect the performance of Pressure Relief Valve (PRV). "How "backpressure" affect PRV performance ? Read more in "Several Impact of Backpressure on Conventional PRV".

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posted by Webworm, 1:46 PM


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